The invention relates to a circuit for producing a periodic sawtooth shaped signal having a trace and a retrace, comprising an oscillator which is directly synchronizable by synchronizing pulses having a substantially constant repetition frequency, a transmission switch for applying said synchronizing pulses to said oscillator, the natural frequency of said oscillator being approximately equal to the said repetition frequency in the absence of a synchronizing signal and being capable of being modified on the presence of a synchronizing signal to a value which is lower than the said repetition frequency, direct synchronisation taking place at the occurrence of a synchronizing pulse in the vicinity of a maximum value of the sawtooth signal.
Phase control loops in which the phase difference between the reference signal produced by an oscillator and the incoming synchronizing pulses is determined by a phase discriminator are of rather common usage in synchronizing circuits, for example for the line synchronization in television receivers. The output voltage of the phase discriminator is smoothed and the voltage obtained continuously adjusts the frequency and/or the phase of the oscillator in such manner that after some time, the so-called lock-in period, the phase difference becomes very small. Such a method is known as "indirect synchronisation". In this method the oscillator is always supplied with a control signal, whereas the synchronizing pulses never reach the oscillator directly.
At low frequencies, for example the field frequency in television receivers, so for which the repetition frequency of the synchronizing pulses is 50 or 60 Hz, a phase control is generally not used. One reason therefor is that such a control circuit is difficult to realize as, at a field frequency of 50 Hz, the control voltage must remain constant for at least 20 ms. It has further been found that the very long time constant of the smoothing filter, this long time constant being required for a proper noise insensitivity, is a source of all kinds of annoying slow phenomena. In practice, preference is therefore given to so-called "direct synchronisation circuits", in which the synchronizing pulses are directly applied to the oscillator to synchronize it.
Applicants' Netherlands Patent Application No. 7811597 (PHN 9287) discloses a sawtooth generator which is directly synchronizable and the natural frequency of which is in the absence of synchronizing pulses substantially equal to the nominal repetition frequency of these pulses. An advantage thereof is that the signal produced by the circuit, for example for the vertical deflection of a picture display tube in a television receiver, may be set nominally, that is to say to the frequency of the received synchronizing pulses, which results in a somewhat stable picture in the absence of these pulses.
The known circuit is symmetrically synchronisable, which means that synchronisation is possible when the natural frequency is somewhat higher or somewhat lower than the frequency of the synchronizing pulses. A slight deviation with respect to the nominal value may even already occur due to tolerances or temperature effects. So, when a synchronizing pulse occurs shortly before or after the final instant of the trace which would be obtained in the absence of synchronizing pulses the retrace is then immediately initiated. For this purpose the natural frequency of the oscillator is switchable to a value which is lower than the nominal frequency. If, however, the synchronizing pulse occurs a longer period of time before the said instant, the lock-in period of the oscillator is then rather long as the phase shift of the pulse with respect to the edge of the sawtooth varies very slowly because of the fact that the frequencies are almost equal, and as in view of the desired noise insensitivity the synchronising pulse does not influence the oscillator before the pulse occurs near the initial instant of the retrace of the free oscillation.